For many years, it has been customary to employ printed circuit boards ("PCBs") or printed circuit assemblies ("PCAs") as a medium for mechanically holding electronic components together and for providing electrical interconnections among the components. The earliest PCBs were constructed of an insulating planar substrate (such as a glass fiber/resin combination) upon which was deposited a layer of conductive metal. Most typically, the metal layer coated the entire surface of the substrate and was chemically etched to place a pattern of conductors (or "traces") on the surface. Often, metal layers were provided on both the upper and lower surfaces of the substrate to allow conductors to cross one another without making electrical contact. A plurality of mounting holes or "vias" were drilled through the metal layer and substrate. The vias were situated to receive leads from the electronic components.
To complete assembly of a circuit board, the electronic components were placed on the PCB, either by hand or robotic machine, the leads of the components passing through corresponding vias. Lastly, solder connections were made to ensure reliable electrical contact between the components and the traces. Initially, soldering was performed manually. Subsequently, more efficient machine-soldering techniques employing infrared ovens or solder baths were developed to speed manufacture of circuit boards and to ensure higher solder joint reliability.
Under such machine-soldering techniques, the PCB and its components were heated. Solder, under the influence of flux, was caused to contact the board and flowed by capillary action into the vias, yielding a low resistance solder joint when cooled.
As circuit board technology developed, designers began to create circuit boards comprising many alternating substrate and conductive layer pairs, resulting in sandwiched circuit boards that could accommodate a higher component density. Such boards could accommodate ten or more conductive layers.
Designers also recognized that, given the number of layers available, it became advantageous to dedicate one layer to provide power to the components and another layer to serve as an electrical ground for the components. Since most of the components require direct access to either power or a ground, these dedicated layers often extended the full area of the PCB. For this reason, the dedicated power layer was termed a "power plane" and the dedicated ground layer was termed a "ground plane." Vias passed through the power and ground planes, allowing component leads to be soldered thereto as required.
Unfortunately, these designers discovered that the power and ground planes acted as efficient heat sinks during the soldering process, causing the planes to cool much faster than the remainder of the PCB. The planes cooled proximate the vias, such that, when solder was introduced, the temperature of the plane was insufficient to allow the solder to flow freely to create a good joint. Instead, the solder tended to solidify unevenly where the via met the planes, causing a "cold joint" of relatively high resistance and poor mechanical strength.
In response, designers provided a prior art thermal relief pattern about each via adapted to provide a degree of thermal isolation between the plane proximate a given via (within the pattern) and the rest of the plane (without the pattern). The prior art thermal relief pattern consisted of a number of cutouts in the conductive layer surrounding a given via. The cutouts had sharp corners or "discontinuities." In between the cutouts remained relatively narrow intersticial bands of conductive metal. The specific geometry of the prior art thermal relief pattern will be described in connection with, and illustrated in, FIG. 1, to follow.
While the prior art thermal relief pattern was suitable to isolate the plane proximate the via thermally from the surrounding plane, designers realized that the interstitial conductive bands posed a significant limit on the amount of current that could be conducted between the via and the rest of the plane. This current-handling limitation became more frustrating as components grew in their power requirements.
Further, as the electronic components began to operate at higher frequencies (particularly in computer systems, including today's personal computers ("PCs")), the designers found that the discontinuities created edge effects, causing reflection of high frequency currents away from the bands instead of allowing the currents to flow through the bands or, worse yet, constructively superposing frequencies, thereby generating or amplifying still higher frequencies and creating or exacerbating an EMI problem. Further, repeated reflections caused resonances of high frequencies between ones of the discontinuities, creating and amplifying harmonics as a function of the distances between the discontinuities. As operating frequencies of the components rose over time, EMI production and retention grew even more problematical.
Accordingly, what is needed in the art is a thermal relief pattern that is at least as adept at thermally isolating the plane proximate each via from the remainder of the plane, but also a pattern that has improved current-handling and EMI characteristics.